A dialysis machine or control unit comprises a dialyzer, dialysate delivery and waste systems, and blood delivery and return systems. It also can contain means for controlling and detecting pH, conductivity, temperature, and pressure of blood and/or dialysate. Such means may be located upstream or downstream of the dialyzer or both. Examples of sensors include pH, dissolved oxygen, flow switch, temperature, pressure, and conductivity sensors, and optical sensors such as a blood leak detector. Sensors located downstream of the dialyzer are typically, but not exclusively, pressure, flow switch, conductivity and blood leak detectors. As with other components of the downstream dialysate circuit, such downstream sensors can become fouled by the accumulation thereon of proteins and other organic and inorganic matter, which can impair their function and/or cause them to generate false readings. Sensors downstream of the dialyzer are more likely to acquire deposits, due to the relatively higher concentration of proteins and other organics in the spent dialysate. For example, blood leak detectors that are optical sensors, e.g., absorbance or diffraction sensors, when sufficiently fouled, can give false blood leak indications. The resulting halt in the dialysis procedure requires that the operator determine the cause of the alarm and clean the fouled blood leak detector.
Sensors can be cleaned by routine maintenance procedures, such as those described in Johnson, U.S. Pat. No. 4,371,385 (1983). Johnson describes, in columns 9 and 10, bleach and formaldehyde rinses to clean and disinfect the dialysate delivery and waste systems. In addition, the COBE Centrysystem.TM. 3 Dialysis Control Unit Operator's Manual (1992) describes maintenance procedures which include vinegar or acid rinse to control precipitate and bleach (sodium hypochorite) rinse to control organics. Further, the COBE Centrysystem.TM. 3 Disinfection and Cleaning Options (1993) manual describes cleaning procedures such as disinfection with Actril.TM., Renalin.TM. and formaldehyde solutions, and organics control with bleach or TERG-A-ZYME.TM.. As discussed in detail hereinbelow, each of the Johnson and COBE procedures involve cleaning both the dialysate delivery and waste systems. No procedure is provided specifically for the cleaning of sensors downstream of the dialyzer. Routine maintenance procedures that involve both the dialysate delivery and waste systems may not be adequate to clean the more heavily contaminated downstream sensors. Moreover, using the routine procedures on an as-needed basis to correct contaminated downstream sensors can be time-consuming. For example, for a routine bleach cleaning, the procedure can require at least 20 minutes. This can disrupt scheduled machine use.
A downstream sensor can also be cleaned by disassembly of the contaminated sensor, manual removal of the contamination, and reassembly of the sensor. For example, von der Heide et al., U.S. Pat. No. 4,153,554 (1979), describe such a method for a fouled blood leak detector. The detector is described as a photocell with two glass windows. The detector can give false readings of a blood leak in the spent dialysate if the window(s) of the photocell acquire a "film," which interferes with the passage of light. At column 8, lines 30-35, it is stated that this situation can be remedied by chemically removing the films from the windows of the photocell by "physically opening the blood leak detector by removing the top . . . which is held in place by screws . . . , and cleaning the windows . . . through the open top of [the] screen . . . " of the detector. To temporarily avoid disassembly of the dialyzer apparatus and avoid a false reading of the blood detector, von der Heide et al. suggest attenuating the sensitivity of the blood detector by adjusting the blood leak sensitivity limit. The suggestion of van der Heide et al. that an alternative method be used to temporarily avoid disassembly reflects the inconvenience and delay associated with sensor disassembly, cleaning and reassembly.
Therefore, there is a clear need for a rapid, simple procedure for cleaning sensors located downstream from the dialyzer. Preferably, the procedure requires significantly less time than that required for routine or maintenance cleaning procedures, so that disruption of scheduled machine use is minimized. Also, the procedure must be sufficiently rigorous to remove substantially all of the deposits on the components of the downstream dialysate circuit, but must not present a hazard to the next dialysis patient.